For field-grown ‘Valencia’ sweet orange (Citrus sinensis) affected by Huanglongbing [HLB (Candidatus Liberibacter asiaticus (CLas)], trees that displayed more severe HLB symptoms (severe trees) had 74% fruit drop before harvest; however, the drop rate for less symptomatic trees (mild trees) was 45%. For mature fruit (3 weeks before harvest) still attached to the branches, 60% of them from severe trees were “loose fruit” [fruit detachment force (FT) < 6 kgf]. In contrast, only 13% of the attached fruit from the mild trees were loose. Overall, fresh weight and size of loose fruit were lower than “tight fruit” (FT > 6 kgf). Irrespective of the symptom levels of trees, the concentrations of glucose, fructose, and inositol in juice of loose fruit were the same or larger than those of tight fruit, suggesting that the shortage of carbohydrates is not the dominant cause of HLB-associated preharvest fruit drop. Expression levels of the cell wall modification genes encoding cellulase (endo-1,4-β-glucanase), polygalacturonase, and pectate lyase were greater in the calyx abscission zones of loose fruit compared to tight fruit, indicating that cell separation was occurring in the former at the time of collection. No differences in the expression levels of genes encoding the ethylene biosynthesis enzymes, including 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO), and an ethylene-responsive transcription factor 1 (ERF1) were observed in tissues of loose and tight fruit. Interestingly, ACS, ACO, and EFR1 expressions were lower in calyx abscission zones and in leaves of the severe trees compared with those of mild trees, suggesting an ostensible, HLB-dependent reduction in ethylene biosynthesis and/or signaling close to harvest time. However, the role of ethylene in HLB-associated preharvest fruit drop remains to be determined. The results leave open the possibility of early ethylene production and action before the initiation of fruit abscission.
We thank Tony Trama and Taylor S. Livingston for their technical support with carbohydrate analysis and field-related activities.
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